This invention relates to a projection apparatus which uses an optical modulation element of the reflection type to modulate illumination light emitted from a light source and uses a lens to project the modulated illumination light as an image of an enlarged scale.
Conventionally, in order to make display on a large screen possible, a projection apparatus is used wherein illumination light from a lamp is irradiated on an optical modulation element such as a liquid crystal panel on which a pattern corresponding to an video signal inputted to the liquid crystal panel is displayed and reflected light from the liquid crystal panel is projected in an enlarged scale to project the pattern displayed on the liquid crystal panel in an enlarged scale.
Various optical systems for a projection apparatus wherein such a liquid crystal panel of the reflection type as described above is used have been proposed. An optical system for colors which uses a polarization rotating element having a function of rotating the polarization direction of light of a particular wavelength band by 90xc2x0 and a polarization beam splitter (hereinafter referred to as PBS) is superior in both of the contrast and the brightness.
An exemplary one of projection apparatus in which such an optical system as just described is used is shown in FIG. 1. Referring to FIG. 1, the projector apparatus 100 shown includes a lamp 101 for irradiating illumination light and a fly""s eye integrator 102, a PS conversion and multiplexing element 103, a main condenser 104, a field lens 105, a pre-polarizing plate 106, a G polarization rotating element 107 and an incoming PBS 108 disposed in order along a path of the light irradiated from the lamp 101.
In the projector apparatus 100 described above, illumination light emitted from the lamp 101 is uniformized by the fly""s eye integrator 102 so as to have a uniform illuminance distribution. Then, P polarized light of the illumination light is converted into S polarized light by the PS conversion and multiplexing element 103 so that the illumination light thereafter includes only S polarized light. Then, the illumination light is condensed by the main condenser 104 and the field lens 105 and adjusted by the pre-polarizing plate 106 so that it has an adjusted polarization plane. Then, the polarization plane of the light in a wavelength band of green from within the illumination light of the arranged polarization plane is rotated by 90xc2x0 by the G polarization rotating element 107 so that the light is converted into P polarized light. Then, the P polarized light, that is, the light in the wavelength band of green, passes through the incoming PBS 108 while the other S polarized light, that is, the light in wavelength bands of red and blue, is reflected by a reflecting surface 108a of the incoming PBS 108, which is inclined by 45xc2x0 with respect to the light path, so that the advancing direction thereof is changed by 90xc2x0.
The projector apparatus 100 further includes a G-PBS 109 and a first liquid crystal panel 110 disposed in this order along the path of the illumination light which has passed through from the incoming PBS 108 and advanced straightforwardly.
The illumination light having passed through and advanced straightforwardly from the incoming PBS 108 is P polarized light in the wavelength band of green and therefore passes through the G-PBS 109 and is introduced to the first liquid crystal panel 110. The illumination light is modulated and reflected to the incoming direction by the first liquid crystal panel 110. The reflected light reflected by the first liquid crystal panel 110 is S polarized light in the wavelength band of green and is reflected by a reflecting surface 109a of the G-PBS 109, which is inclined by 45xc2x0 with respect to the light path, so that the advancing direction thereof is changed by 90xc2x0.
The projector apparatus 100 further includes a R polarization rotating element 111 and a RB-PBS 112 disposed in this order in the direction of the path of the illumination light reflected by the reflecting surface 108a of the incoming PBS 108, a second liquid crystal panel 113 disposed in the advancing direction of the illumination light having passed through the RB-PBS 112, and a third liquid crystal panel 114 disposed in the advancing direction of the illumination light reflected by a reflecting surface 112a of the RB-PBS 112.
The illumination light reflected by the reflecting surface 108a of the incoming PBS 108 is S polarized light in the wavelength bands of red and blue, and the polarization plane of the light only in the wavelength band of red is rotated by 90xc2x0 by the R polarization rotating element 111. The light of the rotated polarization plane enters the RB-PBS 112.
Of the illumination light entering the RB-PBS 112, the P polarized light in the wavelength band of red passes through the RB-PBS 112 and advances straightforwardly so that it is introduced to the second liquid crystal panel 113. Then, the P polarized light is modulated and reflected to the incoming direction by the second liquid crystal panel 113. On the other hand, of the illumination light entering the RB-PBS 112, the S polarized light in the wavelength band of blue is reflected by the reflecting surface 112a of the RB-PBS 112, which is inclined by 45xc2x0 with respect to the light path, so that the advancing direction thereof is changed by 90xc2x0. Consequently, the S polarized light is introduced to the third liquid crystal panel 114 and is modulated and reflected to the incoming direction by the third liquid crystal panel 114.
The reflected light modulated and reflected by the second liquid crystal panel 113 is S polarized light in the wavelength band of red and is reflected by the reflecting surface 112a of the RB-PBS 112, which is inclined by 45xc2x0 degrees with respect to the light path, so that the advancing direction of the S polarized light is changed by 90xc2x0. Meanwhile, the reflected light modulated and reflected by the third liquid crystal panel 114 is P polarized light in the wavelength band of blue, and passes through the third liquid crystal panel 114 and advances straightforwardly.
The projector apparatus 100 further includes a R polarization rotating element 115 disposed in the advancing direction of the reflected light from the second liquid crystal panel 113 reflected by the reflecting surface 112a of the RB-PBS 112 and the reflected light from the third liquid crystal panel 114 having passed through the RB-PBS 112.
The reflected light from the second liquid crystal panel 113 reflected by the reflecting surface 112a of the RB-PBS 112 is S polarized light in the wavelength band of red, and the polarization plane of the reflected light is rotated by 90xc2x0 by the R polarization rotating element 115 so that the reflected light is converted into P polarized light. Meanwhile, the reflected light from the third liquid crystal panel 114 having passed through the RB-PBS 112 is P polarized light in the wavelength band of blue and passes through the R polarization rotating element 115.
The projector apparatus 100 further includes an outgoing PBS 116 disposed in the advancing direction of the reflected light from the first liquid crystal panel 110 reflected by the reflecting surface 109a of the G-PBS 109 and in the advancing directions of the reflected light from the second liquid crystal panel 113 having the polarization plane rotated by 90xc2x0 by the R polarization rotating element 115 and the reflected light from the third liquid crystal panel 114 having passed through the R polarization rotating element 115.
The reflected light from the first liquid crystal panel 110 reflected by the reflecting surface 109a of the G-PBS 109 is S polarized light in the wavelength band of green and is reflected by a reflecting surface 116a of the outgoing PBS 116, which is inclined by 45xc2x0 with respect to the light path, so that the advancing direction thereof is changed by 90xc2x0. Meanwhile, the reflected light from the second liquid crystal panel 113 having the polarization plane rotated by 90xc2x0 by the R polarization rotating element 115 is P polarized light in the wavelength band of red and passes through the outgoing PBS 116 and advances straightforwardly. Further, the reflected light from the third liquid crystal panel 114 having passed through the R polarization rotating element 115 is P polarized light in the wavelength band of blue and therefore passes through the outgoing PBS 116 and advances straightforwardly.
The projector apparatus 100 further includes a G polarization rotating element 117, an outgoing polarizing plate 118 and a projection lens 119 disposed in this order along the light path in the advancing direction of the reflected light from the first liquid crystal panel 110 reflected by the reflecting surface 116a of the outgoing PBS 116, the reflected light from the second liquid crystal panel 113 having passed through the outgoing PBS 116 and the reflected light from the third liquid crystal panel 114.
The reflected light from the first liquid crystal panel 110 reflected by the reflecting surface 116a of the outgoing PBS 116 is S polarized light in the wavelength band of green and is converted into P polarized light by the G polarization rotating element 117 with the polarization plane thereof rotated by 90xc2x0. Then, the P polarized light is adjusted by the outgoing polarizing plate 118 so that it has an adjusted polarization plane and is then projected in an enlarged scale to a screen not shown by the projection lens 119. Meanwhile, the reflected light from the second liquid crystal panel 113 having passed through the outgoing PBS 116 is P polarized light in the wavelength band of red and passes through the G polarization rotating element 117, whereafter it is adjusted by the outgoing polarizing plate 118 so that it has an adjusted polarization plane and is then projected in an enlarged scale to the screen not shown by the projection lens 119. Further, the reflected light from the third liquid crystal panel 114 having passed through the outgoing PBS 116 is P polarized light in the wavelength band of blue and passes through the G polarization rotating element 117, whereafter it is adjusted by the outgoing polarizing plate 118 so that it has an adjusted polarization plane and is then projected in an enlarged scale to the screen not shown by the projection lens 119.
In this manner, the projector apparatus 100 projects a screen image corresponding to video signals inputted to the first, second and third liquid crystal panels 110, 113 and 114 for the three demultiplex colors of green, red and blue in an enlarged scale to the screen by the projection lens 119.
However, in such a projection apparatus which employs an optical system as described above, the contrast varies significantly depending upon at which places the three liquid crystal panels are disposed. This problem arises from a characteristic of a general PBS.
In particular, a general PBS makes use of interference of a thin film to cause demultiplexing and multiplexing of rays of light. Therefore, for example, the reflection factor of P polarized light which should pass through the PBSs is as high as approximately 10%. Consequently, when OFF light which is unnecessary light which should return to the lamp 101 without advancing to the projection lens 119 after it is reflected by the liquid crystal panels passes as P polarized light through each PBS, it is reflected by approximately 10% thereof by the PBS and comes to the projection lens 119, by which it is projected in an enlarged scale to the screen.
For example, in FIG. 1, the illumination light in the wavelength band of green is introduced as P polarized light to the first liquid crystal panel 110. Then, the reflected light from the first liquid crystal panel 110 includes modulated S polarized light and unnecessary P polarized light, and since the transmission factor of the G-PBS 109 for P polarized light cannot be 100%, approximately 10% of the unnecessary P polarized light, that is, the OFF light, is reflected together with the S polarized light by the G-PBS 109. The OFF light reflected by the G-PBS 109 in this manner comes to the outgoing PBS 116 and is reflected at approximately 10% thereof similarly by the outgoing PBS 116 as well. Then, the thus reflected OFF light comes to the projection lens 119 and is projected to the screen.
Such projection of OFF light which is unnecessary light deteriorates the contrast.
An actual contrast where the incoming PBS 108, G-PBS 109, RB-PBS 112 and outgoing PBS 116 have a transmission factor TP for P polarized light of 90%, a reflection factor RP for P polarized light of 10%, a transmission factor TS for S polarized light of 0.5% and a reflection factor RS for S polarized light of 99.5% is calculated below.
With regard to the wavelength band of green, while the illumination light is reflected by the first liquid crystal panel 110 and introduced to the projection lens 119, transmission of P polarized light through the incoming PBS 108, transmission of P polarized light through the G-PBS 109, reflection of S polarized light by the G-PBS 109 and reflection of S polarized light by the outgoing PBS 116 occur, and light to be projected to the screen from the projection lens 119, that is, light on the white side, decreases to TPxc3x97TPxc3x97RSxc3x97RS=80.2%. Meanwhile, while OFF light generated by the first liquid crystal panel 110 is introduced to the projection lens 119, reflection of P polarized light by the G-PBS 109 and reflection of P polarized light by the outgoing PBS 116 occur, and therefore, light which should not be projected to the screen from the projection lens 119, that is, light on the dark side, decreases to TPxc3x97TPxc3x97RPxc3x97RP=0.81%. Therefore, the contrast is approximately 99.
Meanwhile, with regard to the wavelength band of red, while the illumination light is reflected by the second liquid crystal panel 113 and introduced to the projection lens 119, light on the white side decreases to RSxc3x97TPxc3x97RSxc3x97TP=80.2%, and light on the dark side decreases to RSxc3x97TPxc3x97RPxc3x97TS=0.045%. Therefore, the contrast is approximately 1,780.
Further, with regard to the wavelength band of blue, while the illumination light is reflected by the third liquid crystal panel 114 and introduced to the projection lens 119, light on the white side decreases to RSxc3x97RSxc3x97TPxc3x97TP=80.2%, and light on the dark side decreases to RSxc3x97RSxc3x97TSxc3x97TS=0.0025%. Therefore, the contrast is approximately 32,000.
As described above, in the projector apparatus 100 described above which uses an optical system, the wavelength bands of blue, red and green have contrasts which decrease in this order, and the wavelength band of green which contributes most to the contrast of the screen image to be projected exhibits a state of the lowest contrast.
In order to compensate for the drawback described above, the projector apparatus 100 is configured such that the G polarization rotating element 117 is disposed adjacent the projection lens 119 with respect to the outgoing PBS 116 so that the polarization planes of rays of light in the wavelength bands of red, green and blue are adjusted to each other and the outgoing polarizing plate 118 is disposed next to the G polarization rotating element 117 to absorb and purify OFF light which deteriorates the contrasts of the light in the wavelength bands described above.
However, since the G polarization rotating element 117 and the outgoing polarizing plate 118 are additionally provided as described above, the number of devices involved in the optical system is greater as much, and this gives rise to a problem that the transmission efficiency is deteriorated, resulting in drop of the brightness of the screen image. Further, the G polarization rotating element 117 is a very expensive optical system, and employment of the G polarization rotating element 117 increases the cost of the entire projector apparatus.
It is an object of the present invention to provide a less expensive projection apparatus of a high performance which can project a screen image of an improved contrast at a minimized cost using a minimized number of expensive optical elements.
In order to attain the object described above, according to the present invention, there is provided a projection apparatus, comprising a light source for emitting illumination light, a condensing lens for condensing the illumination light emitted from the light source, a first polarizing plate for adjusting polarized light components of the illumination light having passed through the condensing lens, a first polarization rotating element for rotating the polarization plane of the illumination light in the wavelength band of green from within the illumination light whose polarized light components have been adjusted by the first polarizing plate, demultiplexing means for reflecting the illumination light in the wavelength band of green whose polarization plane has been rotated by the first polarization rotating element and passing the illumination light in wavelength bands of red and blue therethrough, a second polarization rotating element for rotating the polarization plane of the illumination light in a first one of the wavelength bands of red and blue from within the illumination light having passed through the demultiplexing means and passing the illumination light in a second one of the wavelength bands of red and blue therethrough, a first optical modulation element for modulating and reflecting the illumination light in the second wavelength band having passed through the second polarization rotating element, a second optical modulation element for modulating and reflecting the illumination light in the first wavelength band whose polarization plane has been rotated by the second polarization rotating element, a third optical modulation element for modulating and reflecting the illumination light in the wavelength band of green reflected by the demultiplexing means, a first polarization beam splitter interposed between the second polarization rotating element and the first and second optical modulation elements for passing the illumination light in the second wavelength band having passed through the second polarization rotating element so as to be introduced to the first optical modulation element and reflecting the illumination light in the first wavelength band whose polarization plane has been rotated by the second polarization rotating element so as to be introduced to the second optical modulation element and for reflecting the modulated and reflected light in the second wavelength band from the first optical modulation element and passing the modulated and reflected light in the first wavelength band from the second optical modulation element therethrough, a third polarization rotating element for passing the reflected light in the second wavelength band reflected by the first polarization beam splitter therethrough and rotating the polarization plane of the reflected light in the first wavelength band having passed through the first polarization beam splitter, a second polarization beam splitter interposed between the demultiplexing means and the third optical modulation element for reflecting the illumination light in the wavelength band of green reflected by the demultiplexing means so as to be introduced to the third optical modulation element and passing the modulated reflected light in the wavelength band of green from the third optical modulation element therethrough, a third polarization beam splitter for reflecting the reflected light in the second wavelength band having passed through the third polarization rotating element, reflecting the reflected light in the first wavelength band whose polarization plane has been rotated by the third polarization rotating element and passing the reflected light in the wavelength band of green having passed through the second polarization beam splitter therethrough, and a projection lens for projecting the reflected light in the first wavelength band reflected by the third polarization beam splitter, the reflected light in the second wavelength band reflected by the third polarization beam splitter and the reflected light in the wavelength band of green having passed through the third polarization beam splitter so as to form a screen image of an enlarged scale.
In the projection apparatus of reflection type according to the present invention, illumination light emitted from the light source is condensed by the condensing lens, and the polarization directions of the illumination light are adjusted by the first polarizing plate. Then, the polarization plane of the illumination light in the wavelength band of green is rotated by the first polarization rotating element, and then the illumination light is demultiplexed into the light component in the wavelength band of green and light components in the wavelength bands of red and blue by the demultiplexing means. Thereafter, the illumination light in a first one of the wavelength bands of red and blue is introduced to the first light modulation element by the second polarization rotating element and the second polarization beam splitter while the illumination light in a second one of the wavelength bands of red and blue is introduced to the second light modulation element. Then, reflected light in the first wavelength band modulated and reflected by the first light modulation element and reflection light in the second wavelength band modulated and reflected by the second light modulation element are introduced to the projection lens by the third polarization rotation element and the third polarization beam splitter so that they are projected in an enlarged scale by the projection lens. Meanwhile, the illumination light in the wavelength band of green is reflected by the second polarization beam splitter and introduced to the third light modulation element. Then, reflected light modulated and reflected by the third light modulation element passes through the second polarization beam splitter and the third polarization beam splitter and is projected in an enlarged scale by the projection lens. Where the third light modulation element for the wavelength band of green is disposed in this manner, the contrast of a screen image can be improved.
In particular, in the projection apparatus, the light modulation element for the wavelength band of green which influences most upon the contrast of the screen image is disposed at a position most advantageous for the contrast, that is, at a position at which the reflected light from the light modulation element for the wavelength band of green is not reflected by any polarization beam splitter until it comes to the projection lens. Consequently, otherwise possible mixture of OFF light into the reflected light from the light modulation element for the wavelength band of green can be prevented and therefore the contrast of the entire apparatus can be improved. Further, the specified optical design of the projection apparatus can reduce the number of expensive optical elements such as a polarization rotating element or a polarizing plate which are required in a conventional optical system. Therefore, the transmission factor is improved and the brightness of the entire apparatus is increased, and the cost can be reduced.
Preferably, the demultiplexing means is formed as a flat plate. This can reduce the cost and the weight of the overall apparatus.
Preferably, the first polarization beam splitter and the third polarization beam splitter are adhered to each other with the third polarization rotating element interposed therebetween, and the second polarization beam splitter and the third polarization beam splitter are adhered to each other. Where the beam splitters are adhered to each other with no air gap left therebetween in this manner so as to form a unitary member, miniaturization of the apparatus can be anticipated and reduction of the cost can be anticipated. Further, where the beam splitters are formed as a unitary member, otherwise possible pixel displacement between the light modulation elements can be suppressed.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.